1. Field of the Invention
The present invention relates in general to exhaust systems of automotive engines, and more particularly to a flexible pipe unit installed in an exhaust pipe line of the automotive engine for absorbing or compensating an inevitable dimensional change of the exhaust pipe line during practical use of the same. More specifically, the present invention is concerned with a flexible pipe unit interposed between the front and rear pipe parts of an exhaust pipe of the automotive engine for compensating a relative displacement between the two pipes.
2. Description of the Prior Art
In order to clarify the task of the present invention, some of conventional flexible pipe units of the above-mentioned type will be briefly described in the following with reference to FIGS. 34, 35 and 36 of the accompanying drawings.
Referring to FIG. 34, there is shown a conventional flexible pipe unit which is shown in Japanese Utility Model Provisional Publication 61-187916. The unit of this publication comprises a metal bellows 100 having axial ends connected to front and rear pipe parts 102 and 104 of an exhaust pipe of an automotive engine (not shown) respectively.
For ease of understanding, the front and rear pipe parts 102 and 104 of the exhaust pipe will be designated by front and rear exhaust pipes in the following.
An outer cover 106 of copper mesh houses therein the bellows 100 having axial ends thereof slidably put on axial ends of the bellows 100 respectively. Protectors 108a and 108b of metal are spot-welded to the front and rear exhaust pipes 102 and 104 in a manner to cover and protect the ends of the outer cover 106. When the unit is subjected to a dimensional change of the exhaust pipe line, the bellows 100 absorbs the change. During this, the axial ends of the outer cover 106 slide on the ends of the bellows 100 thereby having no effect on the outer cover 106.
In FIG. 35, there is shown another conventional flexible pipe unit which is shown in Japanese Utility Model Provisional Publication 2-14418. The unit of this publication comprises a metal bellows 100 having first and second metal collars 110a and 110b welded to both ends thereof respectively. The first and second end collars 110a and 110b are to be welded to the front and rear exhaust pipes 102 and 104 respectively. A plurality of spring wires 112 are arranged to extend between the first and second end collars 110a and 110b. When the unit is subjected to a dimensional change of the exhaust pipe line, the bellows 100 absorbs the change. During this, the spring wires 112 bias the first and second metal collars 110a and 110b toward each other.
In FIG. 36, there is shown still another conventional flexible pipe unit which is shown in Japanese Utility Model Provisional Publication 4-6523. The unit of this publication comprises a metal bellows 100 having axial ends connected to the front and rear exhaust pipes 102 and 104 respectively. An outer cover 106 of stainless mesh houses therein the bellows 100. A rear axial end of the outer cover 106 is welded to the rear exhaust pipe 104, while a front axial end of the outer cover 106 is slidably put on the front end of the bellows 100. The front axial end of the outer cover 106 has a spring retainer 114 connected thereto. The spring retainer 114 has an annular flange part received in an annular housing 116 secured to the front exhaust pipe 102. A plurality of compression springs 118 are received in the annular housing 116, which are compressed between the flange part of the spring retainer 114 and one wall of the annular housing 116. When the unit is subjected to a dimensional change of the exhaust pipe line, the bellows 100 absorbs the change. During this, the front axis end of the outer cover 106 slides on the fixed front end of the bellows 100 thereby having no effect on the outer cover 106.
However, due to inherent construction, the above-mentioned conventional flexible pipe units have various drawbacks which are as follows.
That is, the unit of FIG. 34 is costly due to usage of the outer cover 106 made of copper mesh. Furthermore, for the same reason, the unit is heavy in weight, which induces the need of usage of robust and thus costly holder devices through which the exhaust pipe line is supported from a vehicle floor. Furthermore, as is shown by the graph of FIG. 21, the unit fails to exhibit a satisfied dimensional change compensating performance. In fact, the unit tends to show unstable compensating performance when it is expanded. In the unit of FIG. 35, the bellows 100 and the spring wires 112 are bared and thus tend to be deformed or damaged by foreign objects such as stones during running of an associated motor vehicle. The unit of FIG. 36 is costly due to usage of the outer cover 106 which is made of stainless mesh. Furthermore, due to usage of various and numerous parts, the unit is complicated in construction and heavy in weight.